Common medium multichannel exchange system



Dec. 22, 193. E. l. GREEN r AL,

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Patented Dec. 22, 1935 Fries COMMON MEDI'EJM MULTICHANNEL EXCHANGE SYSTEM Application May 26, 1934, Serial No. 727,805

18 Claims.

This invention has for its object the provision of an exchange system in which communication between subscribers is carried out over channels of different frequency derived. from a common transmission medium. More particularly, the invention is designed to utilize as a substitute for the many subscriber pairs which are employed in the ordinary telephone exchange system the many channels which are obtainable in the highfrequency art as it is now unfolding and to provide means whereby these channels may be selected by subscribers at will and employed for interconnection.

In the telephone exchange systems which have heretofore been provided it has been the pracanother line which extends either directly or via another office or offices to the called party. For a system of this kind many thousands of subscriber circuits are necessary in each central ofhce area. These are commonly provided in the form of a network of multiple pair cables extending over the area.

In accordance with the present invention it is proposed to substitute for such individual wire circuits extending to each subscriber a transmissicn medium which is common to all subscribers and which will accommodate a range of frequencies sufficiently wide for a large number of telephone channels. Interconnection between subscribers is then carried out over these highfrequency channels.

The essence of the invention resides in making available to a large number of subscribers a transmission medium capable of accommodating a wide range of frequencies so that the subscribers may utilize, fer the purposes of intercommunication, a plurality cf signaling channels derived from the wide-band medium. A transmission medium capabie of handling the large number of channels required for the exchange system of the invention may be obtained by utilining either radio transmission in space or highfrequency transmission over a conducting or guiding medium.

Considering, first, the use of radio as the medium for transmitting the wide band of frequencies contemplated by the invention, the invention is designed to employ whatever part of the frequency range may be available. Heretofore, however, the radio-frequency spectrum has scarcely afforded sufiicient frequency space to yield the channel capacity required for a local telephone exchange system. With the opening up of the field of ultra-short waves this limitation as to channel capacity is removed. Thus, for example, between one and two meters there is a total frequency range of 150 megacycles while the range between 10 and 20 centimeters is 1500 megacycles. The frequency ranges obtainable through the use of still shorter waves are still greater.

With these ultra-high frequencies, however, nature imposes another limitation, i. e., the tendency of the waves to take on quasi-optical characteristics, so that they project only in straight lines and do not bend around corners to any considerable extent. Thus, it becomes necessary that subscribers utilizing such waves have a fairly clear line of sight between their antennas. Some diffraction around obstacles may, of course, be obtained, but, in general, reasonably clear line-of-sight projection without intervening obstacles appears to be called for. This, of course, has the advantage of making it possible to utilize the same ultra-high frequencies in different areas.

The line-of-sight characteristics of the ultrashort waves must be recognized in the design of a telephone exchange system utilizing this technique. In one form of the invention it is proposed to provide in the center of a community a repeating station with antennas mounted sufficiently high above the surrounding buildings to make possible the necessary directness of trans-, mission between these antennas and practically any point in the territory to be served. One subscribers station is then placed in connection with another by transmitting first to the central repeating station and having the repeating sta-' tion repeat the transmission to the desired other subscriber located in another part of the community. The central repeating station acts as a means for catching the waves from one subscriber at a relatively high point and transmitting them down again to a second subscriber, thus lifting the transmission path above any obstacles lying between the subscribers antennas.

In order that such a repeating station may be economical, it is proposed to make it capable of handling simultaneously a wide band of frequencies such as would accommodate hundreds or perhaps thousands of channels. The particular arrangement of the repeating station will be hereinafter described.

As an alternative to the use of a radio transmission medium, the invention contemplates the employment of a guiding or conducting transmission path suitable for the required band of frequencies. Such a transmission path would be provided in the form of a network, extending to all the subscribers within a given area. Conceivably, such a network might comprise ordinary wire circuits branched and interconnected so as to make the common circuit available to each subscriber. Thus, the wire network might be somewhat similar to the network employed for distributing light and power currents, except that a simple two-wire network without multi-phase connections, voltage transformation arrangements, etc., would suffice. It would, however, be possible to carry out the invention by employing the light and power network itself and this is contemplated within the scope of the invention.

The preferred form of guiding transmission medium, however, is one which is designed for the transmission of a wide band of frequencies with comparatively low attenuation and preferably also with shielding to minimize the effect of external disturbances. Such a transmission medium may be found in a circuit consisting of two conductors disposed coaxially with respect to one another. In a properly designed coaxial circuit, as will be hereinafter explained, low attenuation at high frequencies may be attained by the employment of conductors of suitably low high-frequency resistance and the use of a substantially gaseous dielectric between the conductors. With this type of circuit the outer conductor serves also as a shield whose protective effect becomes more nearly perfect as the frequency is increased, so that at high frequencies practically complete immunity from external disturbances may be obtained.- r

The invention contemplates also as an alternative to the. coaxial circuit a high-frequency transmission medium comprising a circuit consisting of two conductors surrounded by a shield. Here, again, the high-frequency attenuation may be minimized by proper design with substantially gaseous insulation and the thickness of shield may be determined so as to minimize interference from external disturbances.

Yet another form of transmission medium suitable for carrying out the invention is a dielectric Wave guide comprising a cylindriform dielectric material which may or may not be surrounded by a conductor. An advantageous form of such a transmission path is a hollow cylindrical conductor containing air or some other gas as the dielectric. This type of circuit is particularly adapted to transmit waves of very high frequencies, for example, of the order of a few centimeters or less in wavelength.

Using any of the above types of transmission media, the interconnection of subscribers at will from the available channels involves a number of new and diflicult problems which are solved by the various features of the invention as hereinafter disclosed.

It is contemplated in the present invention that subscribers will be connected together directly by means of the common medium, without necessitating the interposition of a central oflice. Direct connection between subscribers is effected by a process of tuning or selection. This method takes advantage of the fact that the transmission medium is common to all of the subscribers, with all of the channels accessible to each subscriber at will. The switching function is placedin the hands of the individual subscribers by eliminating the central oflice in so far as connections within the given area are concerned. Connection to subscribers in other areas is carried out through the respective central ofiices which are joined by trunk channels or circuits suitable for the purpose.

The invention contemplates the assignment to each subscriber of a particular frequency which might preferably be related to his telephone number. The assignment of frequencies should be such as to facilitate the interconnection of subscribers.

One of the features of the invention is that communication between two subscribers is accomplished using a single carrier frequency. Another feature of the invention is that the carrier frequencies required at the subscribers stations are supplied from a central point. This obviates the necessity of generating carrier frequencies at any subscribers station, and makes possible better frequency stability and the two-way use of a single carrier frequency without interference.

The foregoing outline having indicated some of the principles of the invention as well as a few of its major objects, the complete invention with its various details, features and purposes will now be understood from the following description when read in connection with the accompanying drawing, in which Figure 1 shows a schematic representation of the principal idea of the invention wherein a large number of stationsare connected to a common transmission medium; Fig. 2 shows an embodiment of the invention in which communication is established between subscribers using the same frequency band for both directions of transmission; Fig. 3 shows an antenna arrangement which may be used with subscriber stations of the type shown in Fig. 2; Fig. 4 is a schematic diagram indicating how connections may be established directly between subscribers in the same area, while connections between subscribers in different. areas are completed through central ofiices over trunk circuits; Fig. 5 is a schematic diagram of a guiding or conducting network which is. available in common to a central station and a number of subscribers; Fig. 6 shows a coaxial conductor network for use with the arrangement of Fig. 2, in which amplification is provided at the central station.

Referring to Fig. 1, there is illustrated one of the principal aspects of the invention. In this figure is shown a common medium CM, to which is connected a plurality of stations S1, S2, S3, etc., each of which includes transmitting, receiving and associated apparatus. This apparatus, which will be hereinafter described, is arranged to permit intercommunication between different pairs of stations over the common medium CM employing different frequency bands to permit simultaneous communication between different pairs of stations.

When, as in the present invention, the sam frequency is employed for both directions of transmission, there might be some difficulty in obtaining suificiently close synchronism between the frequencies generated by the two subscribers. Accordingly, it may be desirable in this case for both subscribers to obtain the carrier frequency from a common source. It is proposed that this commonsource be a central station from which there are radiated continuously the frequencies required by the various subscribers. Each subscriber will then select from the frequencies received by his antenna the particular one which he desires to use both for modulation and demodulation. Since the received energy may be small, it may be found desirable to amplify the carrier before applying it to the modulator. An arrangement of this kind is illustrated in Fig. 2, which is a diagrammatic representation of one possible arrangement of subscriber equipment, wherein the same carrier frequency is used for both directions of transmission. In accordance with this arrangement it is proposed that a single carrier frequency be assigned to each subscriber for use on incoming calls and that in placing an outgoing call the subscriber will select the frequency assigned to the called party.

Referring to Fig. 2, the carrier frequency to be used by the subscriber is selected either by the fixed selecting circuit FF1 or by the variable selecting circuit VF1 and led via an auxiliary path to the modulator M02. This modulator is supplied with a local carrier frequency by the oscillator V0. The value of the locally supplied frequency is so determined that the beat frequency produced between it and the incoming carrier corresponds with the pass frequency of the sharp selecting filter FFs. In the output of this filter the beat frequency is amplified by the amplifier AM and applied to the modulator M03 which is supplied also with the frequency from the oscillator V0. Accordingly, there is obtained in the output of the modulator M03 an amplified carrier of precisely the same frequency as that received. This frequency is selected by selecting circuit FFZ or VFz, as the case may be, the unwanted products of modulation being rejected, and the carrier is then applied to the two-way modulator M01. This modulator, being of the balanced type, does not permit the carrier to pass back to the input of the modulator M02, so that singing is prevented.

The operation of the arrangement of Fig. 2 in receiving an incoming call will first be described. In this case, the output of the fixed selecting circuit FF1 (which is tuned to the frequency assigned to the subscriber) is connected to the two-way modulator M01 which serves to modulate both the outgoing signals and the incoming signals. The output of the selecting circuit FF1 is applied also to the modulator M02. The frequency of the oscillator V0 is at this time determined by the fixed condenser FC and is of such value as to combine with the carrier selooted by FF1 to produce the beat frequency which passes FF3. The amplified intermediate frequency is remodulated to obtain the original carrier, FF'2 being adjusted to the same frequency as FF1. The amplified carrier is delivered to M01 which is associated with the hybrid coil 1-13 to which the transmitter ST and the receiver SR are conjugately connected, the latter through the high-pass filter HP.

As will be explained later, the equipment of the calling subscriber will at this time have modulated the carrier frequency with a ringing frequency of 10% cycles. This ringing frequency, being demodulated by M01, passes through the iOOQ-cycle filter FF4 and after rectification in the rectifier RFI, serves to energize the relay RL1. This relay closes the circuit of the bell SB which is connected through the break contact of the switchhook SH.

The subscriber answers the call by taking his receiver off the hook, which breaks the circuit of the bell. The make contact of the switchhook also completes a circuit through the winding of the slow-release relay Ella and a make contact of Rh. Thus RLz is operated, whereupon it forms for itself a locking circuit through one of its make contacts and the make contact of the switchhook, so that RLz remains operated until the switchhcok contact is broken. The operation of RLz serves to disable the relay RLs so that the connections to the fixed selecting circuits W1 and FFz and the condenser FC remain unchanged. The operation of RLz also serves to connect Gil-cycle current through a back contact of R113 to the hybrid coil, whence it passes to the modulator M01. At the other subscribers station this SO-cycle current is demodulated and used to cut off the lOOO-cycle ringing current in a manner to be explained below. The frequency of 60 cycles is continuously applied to the modulator throughout the connection, and serves as a busy indicator in the manner hereinafter explained. RLZ acts also to complete the circuit of the transmitter ST and the receiver SR. Thus, the equipment is in readiness for the conversation to proceed. At the conclusion of the conversation the subscriber hangs up, whereupon his equipment restores to normal.

The operation of the arrangeiricnt of Fig. 2 when the subscriber places an outgoing call will now be explained. First, the subscriber, by means of the control I-IC, adjusts the frequencies of the variable selecting circuits W1 and VFz to the frequency of the called party. At the same time the frequency of the oscillator V0 is adjusted (by means of the variable condenser VC) so that the beat between this frequency and the fre quency of the called party passes the fixed filter F'Fs. Thus the equipment is in readiness to amplify the carrier frequency of the called subscriber, although the circuit connections at this time are still established with the fixed selecting circuits FF1 and W3 and the condenser FC which are employed for receiving the incoming call.

The subscriber now takes his receiver off the hook. Since there is no incoming call, the received carrier is not modulated by the 1000 cycles and the relay RLl is deenergized. Accordingly, a circuit is completed through slow-release relay RL3, which is connected through a back contact of RLz, a back contact of RL1, and the make contact of the switchhook. Thus PtLg is operated, whereupon it forms for itself a locking circuit through one of its make contacts and the make contact of the switchhook. The operation of RLs establishes the connections to the variable selecting circuits VF1 and VFz and the variable condenser VC.

If at this time the frequency of the called party is in use, this frequency will be modulated by 6G cycles, as has been explained above. In this case the GO-cycle current, after being demodulated by M01, will pass through. the low-pass filter LP and, upon being rectified PUFZ, will energize the relay RLi. This will serve to light the busy lamp BL which is connected through a make contact of RL3, a break contact of RLz, the make contact of RLi, and the make contact of the switchhook. The subscriber may then hang up or wait until the called subscribers frequency becomes idle.

If, at the time RLs operates, the called subscribers frequency is not busy, the operation of RL3 serves to apply to the hybrid coil H13 the ringing frequency of 1000 cycles, the connection being carried through a back contact of R114.

The IOOO-cycle frequency passes through the filter FF4 and after rectification energizes the relay RL1, which, in turn, completes the circuit of RL2. In the modulator M01 the ringing frequency is modulated to produce side frequencies of the called partys carrier. At the called station these side frequencies are demodulated and the resultant 1000-cycle current applies a frequency of 60 cycles to the modulator of the called subscriber. The modulated frequencies corresponding to 60 cycles which are sent out from the called station are demodulated by M01 and serve to energize the relay RL4 which cuts out the 1000- cycle ringing current. Since the operation of RLz has completed the circuits of the transmitter ST and the receiver SR, the equipment is now in readiness for conversation. At the conclusion of the conversation, the operation of the switchhook causes the equipment to restore to normal.

One or two further points in connection with the arrangement of Fig. 2 may now be noted. The transmission from one subscriber to another may be accomplished either directly or via the antenna of the central station, the original source of carrier frequency being at the central station. In case the carrier frequency is received by the subscriber at a sufficiently high level, it may be found possible to use this frequency unamplified for demodulation, modulation and re-radiation, so that the auxiliary circuit for amplifying the incoming carrier frequency might be omitted.

Radio transmission between subscribers who are provided with the station arrangement of the type shown in Fig. 2 may be accomplished directly, using any suitable type of antenna. However, in order to obtain substantially lineof-sight transmission for the ultra-short range of frequencies, the transmission path may be carried over obstacles which intervene between the antennas of individual subscribers by employing an arrangement of the type shown in Fig. 3.

Referring to this figure, the frequencies transmitted by the various subscribers are carried through a central repeating station where they are received on the antennae CA1 and CA2 and reradiated from said antennae. These antennas CA1 and CA2 may be located upon a tall building or some other place having sufiicient elevation. Each subscriber's antenna may be designed to radiate a. beam toward the antennae CA1 and CA2 and to receive radiation from said antennae. Antennae CA1 and CA2 also radiate all the carrier frequencies which are needed by the subscribers in the area, which are generated by the apparatus CO.

As has been explained, the subscriber station arrangement of Fig. 2 is designed to provide a direct radio connection between the subscribers within a given area. It is proposed to interconnect such stations with subscriber stations in other areas by providing in each area a central ofiice, so that a radio connection may be established between a subscriber and his central office, using one of several frequencies assigned for that purpose, and the connection completed over suitable interolfice trunks. The method is illustrated schematically in Fig. 4. The central office apparatus to be used in carrying out this method might be patterned after that shown in Fig. 2. The types of trunks which may be used to interconnect the central offices are well known in the art.

In the arrangements which have been described for selecting channels, the entire selection has been accomplished at the channel frequency. It will be evident that this method of scribers equipment maybe incorporated in any of the subscribers station arrangements which have been described, so that conversation between diiferent subscribers may take place over a substantially constant transmission equivalent.

It will be noted that in the subscriber station arrangement described above, the apparatus is designed to prevent a subscriber from listening in on the conversations of other subscribers. This, of course, is a very desirable feature for a telephone exchange system.

In the arrangements described it has been assumed that free space is employed as the common transmitting medium. There will now be described arrangements in which transmission over a high-frequency conducting or guiding path is employed instead of radio transmission.

Such a path would take the form of a network extending to all subscribers within a given area and to the central office for that area.

Probably the simplest form of such a network would be one comprising ordinary two-conductor circuits suitably interconnected and branched so that the common circuit will be available at all desired points. The circuits comprising the network might, for example, consist of pairs of open-wire or cable conductors. A network of this kind is shown schematically in Fig. 5, where each line represents a pair of conductors, C designates the central oince and S a subscribers station.

It is contemplated also in accordance with the invention that the wire network employed for distributing light and power currents might be used as the common medium from which to derive high-frequency channels for the telephone exchange system. The high-frequency channels would be superposed upon the power network by carrier methods which are well known in the art.

Another form of transmission medium, and one which is peculiarly advantageous in that it is capable of transmitting a wide band of frequencies with comparatively low attenuation and which may be so shielded as to be practically immune to external disturbances, may be found in a circuit consisting of two conductors disposed coaxially with respect toone another. Such a circuit has been disclosed in the patents to L. Es-

penschied and H. A. Afiel, No. 1,835,031, Decemshielding becomes more nearly perfect as the.

frequency is increased, so that at high frequencies the noise due to thermal agitation in the conductors becomes the factor which determines the minimum transmission level. 7

As an alternative to the coaxial circuit, the high-frequency transmission medium might comprise a network of circuits, each consisting of two parallel conductors surrounded by a shield. Circuits of this type are disclosed in the patents to Green, Curtis and Mead, No. 2,034,032, Green and Curtis, No. 2,034,033, and Green and Leibe, No. 2,034,034, all granted March 1'7, 1936.

The invention contemplates also utilizing as the high-frequency transmission medium a dielectric wave guide comprising a cylindriform dielectric material which may or may not be surrounded by a conductor. Such dielectric guides are disclosed in the patent applications of G. C. Southworth, Serial No. 661,154, filed March 16, 1933, and Serial No. 701,711, filed December 9, 1933. An advantageous form of such a wave guide is a hollow cylindrical conductor containing air or some other gas as the dielectric. This type of circuit is particularly adapted to transmit waves of very high frequencies, for example, of the order of a few centimeters or less in wavelength.

It is contemplated in accordance with the invention that the subscriber set which has been described may be employed with any of the above types of transmission paths. For this purpose, it is necessary merely to substitute the transmission network for the radio path, replacing the connection of the apparatus to the radio antenna by a connection suitable to the type of medium employed.

Fig. 6 shows an arrangement for using a guiding medium with the subscriber arrangement herein disclosed, wherein coaxial lines extend in several directions from a central point. Each line is terminated in a resistance RT which approximates its characteristic impedance. At the central point each of the inner conductors of the various branches is brought through a resistance JR to a common junction point. The function of the resistance JR is to avoid high-frequency reflections which otherwise would be produced at the junction. The hybrid coil HE is shown connected to the common junction through a resistance JR, though it would be possible to omit this resistance if a suitable value were assigned to the others. The opposite side of the hybrid is terminated in a resistance BR which is equal to JR plus the parallel impedance of all the branches. The various transmissions arriving at the central point are amplified by the amplifier AM and retransmitted.

In addition, the carrier frequencies required by the subscribers are supplied from the source CS at the central point. To use Fig. 6 in conjunction with the subscriber station arrangement of Fig. 2, the antenna connection of Fig. 2 would be replaced by a connection to the coaxial line; It would also be possible to omit the auxiliary circuit shown in Fig. 2 (between lines a-a and bb) for amplifying the carrier.

While the entire preceding discussion has been restricted to systems wherein a common transmission medium is employed for the provision of telephone exchange service between subscribers, it will be understood that the invention contemplates the use of the same general methods for other types of communication. In particular, it will be seen that there has been disclosed a medium which is capable of handling a wide band of frequencies from which a substantial number of television channels may be derived.

It will furthermore be obvious that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated without departing from the spirit of the invention as defined in the following claims.

What is claimed is:

1. In a system for the exchange of intelligence over a common transmission medium between any one and any other of a plurality of stations taken in pairs, the method which consists in transmitting a plurality of controlling frequencies from a common station to all of the other stations, and at any one of said plurality of stations selecting exclusively and utilizing one of those frequencies for two-way communication over said common medium with another station corresponding to the selected frequency.

2. a system for the exchange of intelligence over a common transmission medium between any one and any other of a plurality of stations taken in pairs, the method which consists in transmitting a plurality of controlling frequencies from a common station to all of the other stations, and selecting exclusively at any one of those stations a frequency corresponding to another station and utilizing it for two-way communication between said pair of stations.

3. A system for the interchange of intelligence comprising a central station and a plurality of subscribers stations, a common transmission medium extending to each of said stations, means for transmitting from said central station a pluraiity of carrier frequencies to each of said subscribers stations, and means at each of said subscribers stations for selecting exclusively and utilizing any one of said carrier frequencies for two-way communication with a chosen one of the other subscribers stations, the frequency utilized depending upon the station with which communication is established.

A system for the interchange of intelligence comprising, a central station and a plurality of subscribers stations, a common transmission medium extending to each of said stations, means for transmitting from said central station to all of said subscribers stations a plurality of carrier frequencies, and means at each of said subscribers stations for selecting exclusively and utilizing any one of said carrier frequencies for both transmission and reception in two-way communication with another subscribers station which corresponds to the selected frequency.

5. A telephone exchange system, comprising a central station and a plurality of subscribers stations, a common transmission medium extending to each of said stations, means for transmitting from said central station a plurality of frequencies, and means at each of said subscribers stations for selecting exclusively and utilizing any one of said frequencies to establish a two-way telephone connection directly with any other of said stations, the frequency selected and utilized depending upon the other station with which connection is established.

6. A central station and a plurality of subscribers stations, a common transmission medium extending to all of said stations, means for transmitting from said central station a plurality of frequencies, different ones of said frequencies being individually assigned to said. subscribers stations as called stations, and means enabling any one of said subscribers stations to be operated selectively and exclusively to utilize the frequency assigned to any other subscribers station to establish two-way communication therewith.

'7. A telephone exchange system, comprising a central station and a plurality of subscribers stations, a common transmission medium extending to each of said stations, means for transmitting from said central station a plurality of frequencies, means at each of said subscribers stations for selecting and utilizing any one of said frequencies to establish a two-way telephone connection with any other of said stations, and means for preventing any other station than the two connected from utilizing said frequency after said connection has been established.

8. A plurality of subscriber stations and a centralstation, a common transmission medium extending to all of said stations, means for supplying from said central station a plurality of frequencies, different ones of said frequencies being individually assigned to said subscribers statiom as called stations, means whereby a twoway telephone connection may be established over said medium between any one and any other of said stations, selecting and utilizing for this purpose that one of a plurality of frequencies being radiated by said central station which is assigned to the called station, and means whereby a plurality of such connections may be established and simultaneously maintained.

9. A central radio station and a plurality of subscribers radio stations, means at said central station for radiating a plurality of carrier frequencies corresponding respectively to said subscribers stations, and means at each of said subscribers stations as a calling station for choosing and utilizing any of said carrier frequencies for transmission to and reception from the corresponding called station.

10. A central radio station and a plurality of subscribers radio stations, means at said central station for radiating a plurality of carrier frequencies corresponding respectively to said subscribers stations, and means at each of said subscribers stations for choosing and utilizing any of said frequencies to establish a telephone connection with another of said subscribers stations corresponding to the chosen frequency.

11. A central radio station and a plurality of subscribers radio stations, means at said central station for radiating a plurality of carrier frequencies corresponding respectivel to said subscribers stations, means at said subscribers stations for choosing and utilizing any one of said frequencies to establish communication with the corresponding subscribers station, and means for preventing any other station from utilizing any frequency which is in use for connection between two stations.

12. A central radio station and a plurality 01' subscribers radio stations, means at said central station for radiating a plurality of carrier frequencies, means at each of said subscribers stations for selecting any one of said carrier frequencies and utilizing said selected frequency for transmitting and receiving for establishing communication with a second subscribers station, means to modulate said carrier frequency with a tone, means at each subscribers station responsive to the receipt of tone modulated carrier frequency for preventing the use by the station of a carrier frequency in use.

13. A central station and a plurality of subscribers stations, a common transmission medium extending to each of saidsubscribers" sta tions, means at said central station for radiating a plurality of carrier frequencies, means at each of said subscribers stations for selecting at will and receiving exclusively any one of said carrier frequencies for two-way communication with any selected one of said subscribers stations, means for modulating said received carrier frequency, and means for transmitting the modulated carrier frequency in communicating with the selected station.

14. A central station and a plurality of subscribers stations, means at said central station for radiating a plurality of carrier waves, means at each of said subscribers stations for selecting at will and receiving exclusively any one of said carriers for two-way communication with any selected one of said subscribers stations, means for amplifying said received carrier, means for modulating said carrier, and means for transmitting said modulated carrier in communicating with the selected station.

15. A central station and a plurality of subscribers stations, means at said central station for radiating a plurality of carrier frequencies, means at each of said subscribers stations for selecting at will and receiving any one of said carrier frequencies, means for modulating said received carrier, and means for demodulating received signals with said carrier.

16. A central station and a plurality of subscribers stations, means at said central station for radiating a plurality of carrier frequencies, means at each of said subscribers stations for selecting at will and receiving any one of said carrier frequencies, and a modulator capable of simultaneously modulating said received carrier with other signals and demodulating incoming signals.

17. In a telephone exchange system, a central station and a plurality of subscribers stations, means for radiating from said central station a plurality of carrier frequencies, one of said carrier frequencies being assigned to each of said subscribers stations, means at each of said subscribers stations for selecting at will any one of said carrier frequencies, and means for utilizing said received frequency to establish two-way communication with the subscribers station assigned that frequency.

18. A plurality of stations associated with a common transmission medium, means at each station for transmitting and receiving on any of a plurality of carrier frequencies, each of said ,gations being continuously prepared to receive on a different carrier frequency, means at a calling station for transmitting a tone modulated on a carrier frequency for actuating a signal at a called station, and means at a called station for transmitting back to the calling station a different tone modulated on a carrier frequency, means at the calling station responsive to the receipt of said last mentioned tone at said calling station to stop the transmission of said first tone.

ESTILL I. GREEN.

NEWTON MONK.

WARREN H. TIDD. 

